Their report claims that, contrary to popular opinion, Camp 16 is not completely enclosed within a single fixed security fence or wall, and that only the lower third of the camp has a fence.

Based on this recent satellite imagery analysis, the report indicated Camp 16 remains North Korea's largest political prison camp. The camp remains active, even in recent winter imagery, focusing primarily on logging, agricultural fields, orchards, livestock, a few fish farms, mining, light industry and hydroelectric power generation.

The report also claimed there has been an increase in the number of housing units and support buildings.

"Although analytical caution is essential, especially in the absence of direct testimony from former Camp 16 prisoners or guards, the significant expansion of housing units seems to indicate that there has been an increase in the prisoner population," HRNK Executive Director Greg Scarlatoiu said.

"The increase may have been the result of prisoner transfer from facilities that have been closed, such as Camp 22 in Hoeryong near the border with China,” he said. “It also may be the result of more people being imprisoned due to the ongoing crackdown on attempted defections and purging of senior officials."

No witnesses, escapees

The infamous facility is located in North Hamgyong province. It encompasses 53 named "villages" and numerous unnamed "villages" across an area of 539 square kilometers.

Of North Korea's four operational political prison camps, Camp 16 is the only one with no known witnesses or escapees.

"As reports continue to emerge on possible developments at North Korea's Punggye-ri nuclear test facility, it also will be important to keep a watchful eye on Camp 16," said AllSource co-founder and chief analytics officer Joseph Bermudez.

AllSource used pan-sharpened satellite imagery collected by DigitalGlobe and Airbus Defense and Space from April 2013 to January 2015. The report also used a declassified KH-9 satellite image taken in October 1983.

HRNK and AllSource released reports on Camp 14 last year and Camp 25 in June of last year, respectively.

The U.N. Commission of Inquiry on Human Rights in North Korea estimated in its final report last year that there are 80,000 to 120,000 prisoners in four camps in North Korea.